The present invention relates to a gas circuit breaker for interrupting a current which occurs due to a ground fault of a line or a short-circuiting failure between lines, for the purpose of protecting an electricity transmission system or an electricity distribution system, and more specifically to a gas circuit breaker capable of extinguishing an arc by utilizing both of a mechanical compression and a pressure elevation effect caused by the thermal energy of the arc, thereby interrupting a current.
At present, as a breaker for protecting a high voltage transmission system of 72 kV or higher, the puffer type gas circuit breaker made of a simple structure, and having a high reliability and an excellent gas-breaking performance, is widely used. The puffer type gas circuit breaker operates in the following manner. That is, an arc-extinguishing gas such as SF.sub.6 gas is compressed by the movable cylinder which is directly connected to the movable contact, so as to generate a high-pressure gas flow, and the gas flow is blown on the arc, so as to extinguish the arc, thereby interrupting the current. Therefore, the interruption performance is determined by the pressure elevation within the movable cylinder. Therefore, when a high pressure elevation is obtained, a high interruption performance is obtained; however the pressure elevation causes a reaction force of the mechanical driving force. Consequently, high driving energy is required to achieve a high interruption performance.
Under these circumstances, there have been a variety of developments and researches made for producing gas circuit breakers of a high interruption performance, which can obtain a high pressure elevation with small driving energy. An example of such breakers is disclosed in Jpn. Pat. Appln. KOKOKU Publication No. 57-54886 and U.S. Pat. No. 4,139,752. In these documents, the development on the basis of the following method is discussed. That is, a thermal pressure elevation room, the pressure inside of which is elevated as a high-temperature gas flows into the room from an arc, is provided in front of the compression room, and a check valve for inhibiting the gas from flow into the compression room from the thermal pressure elevation room is mounted to the partition wall between the thermal pressure elevation room and the compression room, so as to have both rooms communicated one another. Thus, the flow of the high-temperature gas from the thermal pressure elevation room to the compression room, which occurs when a large current is interrupted, is prevented, so as to maintain the pressure elevation in the compression room at a low rate, thereby decreasing the driving energy.
Further, as an improved version of the above-described technique, which can reduce the driving energy more effectively, a gas circuit breaker as shown in FIG. 1 has been developed. (See Jpn. Pat. Appln. KOKAI Publication No. 7-109744)
The conventional gas circuit breaker will now be described with reference to FIG. 1. FIG. 1 is a cross section of the conventional breaker, the lower half of which indicated by the center line in the figure, illustrates an electrode closing state, and the upper half of which illustrates the state of the completion of the closing operation.
As can be seen in FIG. 1, a stationary contact section 10 and a movable contact section 20 are arranged such as to face each other within a container (not shown) filled with an arc-extinguishing gas. It should be noted that with regard to the position of the movable contact section 20, the stationary contact section 10 side is defined as the forward side, and the opposite side is defined as the backward side, for the sake of the convenience of explanation.
The stationary contact section 10 is made of a stationary arc contact 1 and a stationary conductive contact 2 arranged around the arc contact 1. The movable contact section 20 is made of a hollow operating rod 3 having a flange 3a at its front end portion, a movable cylinder 4 arranged around the operating rod 3 and connected to the flange 3a, a hollow movable arc contact 5 fixed to the movable cylinder 4, and having a plurality of fingers arranged in line along the lateral face of the imaginary cylinder such as to be apart from each other, a movable conductive contact 6 disposed around the arc contact 5, an insulating nozzle 7 surrounding the movable arc contact 5 and a stationary piston member 8 inserted to the rear portion of the movable cylinder 4.
The interior of the movable cylinder 4 is partitioned by a middle partitioned plate 4a into a thermal pressure elevation room S.sub.1 located at the front, and a compression room S.sub.2 at the back. A check valve 16 is provided on the middle partition plate 4a, so as to inhibit the gas flow from the thermal pressure elevation room S.sub.1 to the compression room S.sub.2, and allow the counter gas flow. Between the movable arc contact 5 and the nozzle 7, a gas flow path is provided to guide the gas from the thermal pressure elevation room S.sub.1 to the stationary arc contact 1 side.
In the movable contact section 20, the operating rod 3 is formed to reciprocate in its axial direction as driven by a drive device (not shown), and at the rear position of the operating rod 3, a plurality of exhaustion holes 3b which can make the hollow portion of the rod and the gas-filled atmosphere communicate, are made.
A piston 8a is formed to have a donut-disk shape, the inner circumferential surface of which slides on the outer circumferential surface of the operating rod 3 and the outer circumferential surface of which slides on the inner circumferential surface of the portion of the movable cylinder 4 which forms a compression room space S.sub.2. Here, the piston 8a has a hollow supporting tube 8b provided integrally at the rear portion thereof so as to extend in the axial direction, and the piston 8a is fixed by the supporting tube 8b within a container (not shown) via a supporting insulating member (not shown).
As the operating rod 3 and the movable cylinder 4 moves as an integral unit with relative to the piston 8a fixed as above, the movable cylinder 4 and the piston 8a move relative to each other, and thus the compression room space S.sub.2 created within the movable cylinder 4 is compressed. At the rear portion of the supporting tube 8b, a plurality of exhaust holes 8c which make the hollow portion of the supporting tube and the gas-filled atmosphere within the container communicate to each other, are made.
Further, the piston 8a is equipped with a release valve 18 which limits a pressure elevation in the space S.sub.2 by releasing gas within the compression room space S.sub.2 to the gas-filled atmosphere when the pressure elevation in the compression room space S.sub.2 exceeds a predetermined value during the electrode opening operation which interrupts a large current, and a check valve 17 can prevent the pressure reduction in the compression room space S.sub.2 by allowing the gas to flow from the gas-filled atmosphere to the compression space S.sub.2 during the electrode closing operation.
Further, a plurality of grooves 3d and 3e are made at two locations on the outer circumferential surface of the operating rode 3 by process, to extend in the axial direction. The groove 3d is formed to be situated, for its entire length, within the compression room space S.sub.2 when the electrode is closed as shown in the cross section of the lower half of FIG. 1, and to have the compression room space S.sub.2 communicate to the gas-filled atmosphere when the electrode is opened as shown in the upper half of FIG. 1.
The groove 3e is formed to have the compression room space S.sub.2 and the gas-filled atmosphere communicate to each other when the electrode is closed. The function of the groove 3d is to assure a decrease of the pressure elevation of the compression room space S.sub.2 at the final stage of the electrode opening operation, so as to contribute to the achievement of the lowering the driving energy. The function of the groove 3e is to assure the gas flow to the compression room space S.sub.2 at the final stage of the electrode closing operation.
Next, the operation of interrupting a current by means of the electrode opening operation of the conventional gas circuit breaker shown in FIG. 1 will now be described.
During the electrode opening operation, the operating rod 3 is moved in the direction indicated by arrow D, and therefore the movable section including the operating rod 3, that is, the operating rod 3, the movable cylinder 4 connected thereto, the movable arc contact 5, the movable conductive contact 6 and the nozzle 7 are moved as an integral unit to the direction indicated by arrow D. Thus, the volume of the compression room space S.sub.2 created by the rear portion of the movable cylinder 4, which is defined by the middle partition wall 4a, and the piston 8a, is reduced, and therefore the pressure within the compression room space S.sub.2 is increased. The check valve 16 opens its valve rapidly to follow the accelerated movement of the movable section in the beginning of the electrode opening operation, and thus the open state of the check valve 16 is maintained due to the pressure elevation in the compression room space S.sub.2, which occur from then onward. Therefore, the gas flows from the compression room space S.sub.2 to the thermal pressure elevation room S.sub.1. Consequently, the pressure within the thermal pressure elevation room S.sub.1 is slightly increased, and the gas flows in the direction towards the stationary arc contact 1 through a flow path between the nozzle 7 and the movable arc contact 5.
In the meantime, due to the electrode opening operation described above, first, the stationary conductive contact 2 and the movable conductive contact 6 are separated from each other, and then after some delay, the stationary arc contact 1 and the movable arc contact 5 are separated from each other. Thus, an arc is generated between the arc contacts 1 and 5. When the interruption current is as small as about 1 kA or less, the pressure elevation in the thermal pressure elevation space S.sub.1 due to the interruption current is so low that the gas flow state from the compression room space S.sub.2 to the thermal pressure elevation room S.sub.1 is maintained. Consequently, the gas is blown to the arc, thereby causing the interruption.
By contrast, when a large current of about several tens of kilo amperes is interrupted, the high-temperature gas from the arc flows reversely in the flow path between the nozzle 7 and the movable arc contact 5, and enters the thermal pressure elevation room space S.sub.1 so as to heat the gas within the thermal pressure elevation room space S.sub.1 thus elevating the pressure to a high value. Due to the high pressure, a gas flow is created from the nozzle 7 towards the stationary arc contact 1 to cool down the arc, and the arc is extinguished finally at the zero point of the alternating current wave, where the interruption current becomes zero.
When the pressure of the thermal pressure elevation room space S.sub.1 is raised high, the check valve 16 is closed and the gas flow from the thermal pressure elevation room space S.sub.1 to the compression room space S.sub.2 is inhibited. Therefore, the pressure elevation in the compression room space S.sub.2, which is caused by the flow-in of the high temperature gas, is prevented.
However, at the same time, the flow-out of the gas from the compression room space S.sub.2 to the thermal pressure elevation space S.sub.1 is ceased. Therefore, the pressure elevation in the compression room space S.sub.2 becomes significantly high as compared to the pressure elevation which occurs in the electrode opening operation with no load or in the electrode opening operation for interrupting a small current. However, at this time, the release valve 18 operates so as to keep the pressure elevation in the compression room space S.sub.2 at a predetermined low value. Further, at the final stage of the electrode opening operation, the compression room space S.sub.2 communicates to the gas-filled atmosphere via the groove 3d as can be seen in the cross section of the upper half of FIG. 1, thus assuring a decrease in the pressure elevation value in the compression room space S.sub.2. In this manner, the interruption of a large current and the lowering of the drive energy are achieved.
However, such a conventional gas circuit breaker as described above, has characteristics as shown in FIG. 2, that is, in order to interrupt a large current caused by a short-circuiting accident, when the current value becomes low as it goes beyond the vicinity of a peak, the pressure elevation value decreases steeply, and the pressure elevation value at the current zero point significantly decreases as compared to that at the peak of the pressure elevation value. The characteristics described here are discussed in thesis CIGRE-13-110-1994-P6-FIG. 11. A significant decrease in the pressure elevation is a phenomenon which occurs inevitably in the thermal pressure elevation room space S.sub.1, which has no compression effect, and the phenomenon is caused by the ceasing of the flow of the high-temperature gas from the arc to the thermal pressure elevation room space S.sub.1, which occurs when the current value is decreased, or by the rapid reduction of the volume of the high temperature gas located close to the arc.
Apart from the above, it is necessary to obtain a high pressure elevation at the zero current point, for achieving a high interruption performance. Therefore, the reduction of the pressure at the current zero point becomes more significant as the arc time is prolonged. Thus, it is difficult to maintain a high interruption performance. When the peak of the pressure increase value is increased, a high interruption performance can be maintained. However, it is clear that such a method would increase the reaction force to the driving force, and therefore it is not efficient.
Further, the pressure elevation in the thermal pressure elevation room space S.sub.1 at the interruption of a large current is achieved not by an increase in the density, caused by the compression and/or the flow of the gas from the compression room chamber S.sub.2, but by an increase in the temperature, caused by the high temperature gas from the arc. Consequently, when the gas flows out of the nozzle 7 while the temperature keeps on increasing after the interruption of the current, and the pressure decreases to substantially the same pressure of the gas-filled atmosphere, the gas density of the thermal pressure elevation room space S.sub.1 has already decreased significantly to a level lower than the initial value (which is the same as the gas density within the gas-filled atmosphere).
In order to maintain stable power supply after an accident in a power supply system, a gas circuit breaker is required to have a performance of a high speed electrode re-closing interruption, in which the electrode is re-closed immediately after an interruption, and thus another interruption is carried out immediately, as a specification of the device. When the gas density in the thermal pressure elevation room space S.sub.1 is significantly low after an interruption, it is very difficult to obtain a sufficiently high pressure elevation value when a re-interruption is carried out immediately after an interruption. Further, even if the pressure is elevated, a low-density gas is blown to the arc, and therefore the interruption performance is deteriorated. The deterioration of the high-speed electrode re-closing interruption performance is a serious problem, and as a solution, it is required to increase the gas compression cross sectional area of the compression room space S.sub.2 or to increase the driving energy. In the gas circuit breaker, there is an increased amount of load on the damper of the breaker, and therefore the size of the damper is increased.
In general, gas circuit breakers employ a damper operating on oil pressure or the like, for the purpose of decreasing the speed of the movable section immediately before the completion of the electrode opening operation, so that the section can stop at low impact. Although it has been stated above that an excessive pressure increase in a puffer-type gas circuit breaker which compresses the gas with a movable cylinder, is not desirable since it increases the driving energy, as far as the pressure elevation in the compression room immediately before the completion of the electrode opening operation is concerned, it is desirable for the reducing the speed, and further the load on the damper is lightened. In the case of the gas circuit breaker having the structure as shown in FIG. 1, the pressure elevation in the compression room space S.sub.2 is limited by the release valve, and in the final stage, it is further reduced by the groove 3d. Then, at the completion of the electrode opening operation, the pressure elevation becomes substantially zero. Therefore, the speed reduction effect for the movable section, caused by the pressure elevation in the compression room space S.sub.2, is not expected, and therefore the speed reduction must be taken care of only by the damper equipped. As a result, it is necessary to increase the size of the damper.
As described above, in order to solve the problems of the deterioration of the interruption performance and the enlargement of the equipment device, the size of the entire breaker including the driving mechanism must be increased to improve the performance. However, the enlargement of the size of the breaker will result in economical disadvantages in manufacturing and operation of the gas circuit breaker, and therefore it is not desirable.